Cortisol – an important product of the adrenal gland
Apart from various other hormones, the adrenal gland secretes cortisol, a substance known more to many people as “cortisone” from certain medications. The body produces this substance itself, however. Cortisol actually acts in all tissues and organs but particularly influences the immune system as well as glucose and fat metabolism. Production of cortisol is controlled by the higher-level gland, the pituitary, which in turn is stimulated to do so by the hypothalamus. The pituitary produces ACTH (adrenocorticotropic hormone), which then initiates cortisol production in the adrenal gland. When a lot of cortisol is secreted, it reduces the production of ACTH in the pituitary; this is called negative feedback because the cortisol thereby influences its own production.
What is the ACTH stimulation test used for?
Certain diseases disturb the sensitive balance between the hormones. There is then excessive secretion of individual substances, for instance because of a tumour or excessive stimulation of the gland. However, there can also be too little production of important hormones.
In hyperadrenocorticism (also called Cushing’s disease or hypercortisolism) the dog has permanently very raised cortisol levels in the blood. Various disease symptoms result (see there). Cushing’s disease can have different causes. Possible Cushing’s disease and its causes can be investigated more closely with an ACTH stimulation test.
There can also be too little cortisol production and this is called hypoadrenocorticism. This too is revealed with the ACTH stimulation test.
How does an ACTH stimulation test work?
A blood sample is taken twice from the animal at a one-hour interval. To do this, the skin on a fore- or hindleg is usually shaved and disinfected. Blood is then taken from a vein with a sterile needle. After the first sample to measure the cortisol level, a certain amount of ACTH is injected. After an hour, the cortisol level in the blood is measured again.
In a healthy dog, the cortisol level increases moderately in this hour as the ACTH stimulates cortisol production. In an animal with Cushing’s disease, on the other hand, the cortisol concentration in the blood rises much more after the ACTH injection. If the initial cortisol level is very low and does not react to the ACTH injection, this indicates hypoadrenocorticism (also known as Addison’s disease) and thus a disturbance of cortisol production in the adrenal gland.
The ACTH stimulation test is also used to monitor the effect of Cushing’s disease treatment. If it is successful, the second ACTH test level will not rise greatly as it did previously.